Process for burning retorted oil shale and improved combustor

ABSTRACT

Combustor and process for burning particularized oil shale by preburning separate fine and coarse particle feedstreams prior to mixing in the main combustion zone.

BACKGROUND OF THE INVENTION

Following the pyrolysis of oil shale to extract the volatile components,such as shale oil and hydrocarbon gases, a solid material remains whichis referred to as "retorted oil shale". This material contains residualcarbonaceous material which may be burned to yield heat energy. The heatrecovered from the carbonaceous residue may be used to supply heat forthe pyrolysis of the oil shale during the retorting process.

The inorganic residue that remains after the combustion of thecarbonaceous material present in the retorted shale is called "burnedshale". This material is recycled in some retorting processes as "heattransfer material", i.e., the hot burned shale from the combustion ismixed with raw oil shale and the heat provided is used for retorting theraw shale. In U.S. Pat. No. 4,199,432 a process is described in whichthe oil shale is retorted in a downward moving bed containing a mixtureof recycled hot burned shale (used as a heat transfer material) andparticulate raw oil shale. A countercurrent flow of gas passes upwardthrough the bed removing the product vapors and pneumatically entrainingthe finer particulate fraction of the oil shale. The fine particles andproduct vapors are drawn off the top of the retorting vessel, and thefine shale particles are removed from the gas stream in a cyclone.Processes such as this present some problems in designing an efficientcombustor for burning the retorted oil shale which is recycled as theheat transfer material.

During combustion of the carbonaceous residue in the retorted oil shaleto produce heat, the physical integrity of the shale particles ischanged and a substantial amount of fine grained burned shale isproduced which is not suitable for use as recycled heat transferparticles. Therefore, it is necessary to separate this fine materialprior to recycling the coarser grained particles.

In process schemes using a liftpipe combustor to burn the residualcarbonaceous material in the retorted oil shale, sufficient residencetime is required to complete combustion and to assure a thermalequilibrium between the hot burning particles and the cooler recycleparticles. Typically, a minimum residence time of 2 to 3 seconds in thecombustion zone is required. If the fine shale and coarse shaleparticles are combusted in a liftpipe, the pipe must be of sufficientlength to provide adequate residence time for all particles.

The present invention is advantageous for efficiently burningparticulate retorted oil shale where the fine grained material and thecoarser grained material are separated prior to combustion and theburned shale serves as heat transfer material in the retorting process.

SUMMARY OF THE INVENTION

This invention concerns a combustion device for burning particulateretorted oil shale separated into a fine particulate feedstream andcoarse particulate feedstream which comprises:

(a) a generally vertical main combustion chamber having an upper zoneclosed at the top, a middle zone, and a lower zone open at the bottom,said lower zone having means for pneumatically entraining solidsentering said lower zone, said middle zone being an elongate tubularzone adapted for the combustion of entrained particulate solids, saidupper zone being adapted for the disengagement of entrained coarseparticles and for the collection and removal of the disengaged coarseparticles, said upper zone further having an outlet for the removal ofcombustion gases and fine entrained particles;

(b) a generally vertical tubular fine solids combustion chamber adaptedfor burning pneumatically entrained fine solids, said fine solidscombustion chamber having an open upper end communicating with the lowerzone of the main combustion chamber, said fine solids combustion chamberfurther having a lower end having an inlet for the introduction of thefine particulate feedstream and means for pneumatically entraining thefine particulate material and conveying it the length of the fine solidscombustion chamber;

(c) a coarse solids preburning chamber having an inlet for theintroduction of the coarse particulate feedstream, means for fluidizinga burning bed of the coarse particulate material, and means forcommunication with the lower zone of the main combustion chamber.

The present invention is further directed to a process for producingheat from retorted oil shale containing residual carbonaceous material,wherein said retorted oil shale contains both fine and coarse grainedmaterial and at least part of said fine grained material is contained ina separate feedstream from the coarse grained material, said processcomprising:

(a) burning at least part of the residual carbonaceous material in thefine grained material present in said separate feedstream in a finepreburner by entraining said fine particles in a first entraining gasstream containing oxygen and having a velocity greater than the terminalvelocity of the fine particles;

(b) burning part of the carbonaceous residue in the coarse grainedmaterial in a coarse preburner containing less than a stoichiometricamount of oxygen;

(c) mixing the entrained partially burned fine grained material from thefine preburner and the partially burned coarse grained material from thecoarse preburner in a second entraining gas stream having a velocitygreater than the terminal velocity of the mixture of fine and coarseparticles and containing at least a stoichiometric amount of oxygen; and

(d) burning the carbonaceous residue remaining in the entrained fine andcoarse grained material in a vertical combustion zone through which saidsecond entraining gas is directed.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of an oil shale retorting processof the type in which the present invention may be used mostadvantageously.

FIG. 2 illustrates a cross sectional view of a preferred embodiment ofthe combustion device of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, raw crushed oil shale enters retort 2 via conduit 4where it is mixed with the hot heat transfer material (spent shale)entering the retort through conduit 6. The mixture of hot spent shaleand raw shale moves downward in the retort 2 through a series ofdispensing elements 8, 10 and 12 which prevent substantial verticalbackmixing of the solids. A substantially oxygen free retorting gasenters retort 2 through gas inlet 14 creating a countercurrent gas flowthrough the retort in opposition to the downward moving bed of shale.Gas from the gas inlet 14, product vapors, and pneumatically entrainedfine particles of shale move upward and leave the retort via conduit 16.The gas and product vapors are separated from the entrained fines incyclone 18; the gaseous material exits by conduit 20 and the fines arecarried away by conduit 22.

The coarse material containing particles of both burned oil shale andretorted oil shale leaves the bottom of retort 2 via solids conduit 24and is conveyed to the combustor 26. Air entering the combustor via airconduit 28 is mixed with the coarse particles of shale and the residualcarbonaceous material present in the retorted material is ignited. Inthis particular process scheme the fine particulate oil shale materialseparated by cyclone 18 enters the combustor as a separate feedstreamfrom the coarse particulate material from the retort. The advantages ofthis scheme will be discussed in detail later. The fine material carriedby conduit 22 is pneumatically entrained by air from conduit 30 andenters the combustor at the bottom. Following combustion of thecarbonaceous residue the coarse particles of burned shale and fineparticles of burned shale are separated in the top part of the combustorby a process explained in greater detail below. The coarse particles ofburned shale leave the combustor via common conduit 32. Hot burned shaleserving as heat transfer material is recycled to the retort via conduit6 while excess burned shale is drawn off through outlet 34. Flue gasesand entrained fines leave the combustor via conduit 36 and are carriedto cyclone 38. Fines are disposed of through conduit 40. Flue gases arecarried off via conduit 42 for venting or recycling as retorting gas.

Turning to FIG. 2, a combustion device suitable for use in the retortingprocess described above is illustrated in detail. Fine particles ofshale from the retort are carried to the combustor by fine shale inletline 102. Air entering air inlet 104 is distributed around the open end106 of fine shale liftpipe 108 by air plenum 110. The pneumaticallyentrained fines are ignited and carried upward through fine shaleliftpipe 108. The burning fines exit the open upper end 112 of the fineshale liftpipe 108 and enter the liftpipe engaging area 114 for the maincombustion liftpipe 116.

Coarse shale from the retort enters the annular coarse shale preburningarea 118 via inlet 120. A fluidized bed of coarse shale is supported byperforated plate 122 and burned with a fluidizing gas containing asubstoichiometric amount of oxygen introduced via inlet 124. Coarseparticles from the fluidized bed in the preburner 118 spill over intothe liftpipe engaging area 114 of the main combustion liftpipe wherethey, along with fine shale from fine shale liftpipe 108, are entrainedin a gas stream containing excess oxygen entering through primary airinlet 126. Both coarse and fine particles are carried upward by the gasstream and burned in the main combustion liftpipe 116. The burnedparticles of oil shale enter the enlarged upper zone 128 of thecombustion device which serves as a coarse solids disengaging area. Thecoarse particles of burned shale settle out in collection zone 130 whilethe burned fines and flue gases leave the combustion device via outlet132. Coarse particles of burned shale for recycling to the retort aredrawn off via outlet 134. Excess burned coarse shale is removed viaoutlet 136 for disposal.

As used herein the phrase "fine particles of burned shale" refers toparticles of a size unsuitable for recycling as heat transfer material.Usually particles smaller than about 100 to 200 mesh size (Tylerstandard), i.e., about 75 to 100 microns in diameter, are not suitablefor use in the retorting process. Therefore, particles below this rangeare preferably removed with the flue gas as entrained fines. Theseparation of the fine and coarse particles is inherent in the design ofthe upper disengaging area of the combustion device. By exclusion"coarse particles of burned shale" refer to particles larger than about200 mesh size. It should be understood that the terms fine and coarseare relative terms, the size of which may vary somewhat depending on theexact details of the process scheme. Thus in process schemes whereparticles smaller than 200 mesh may be tolerated, the term fine mayinclude particles of a smaller diameter. Likewise, under othercircumstances particles of a larger minimum mesh size may be requiredand the definition of "fine" may be adjusted accordingly.

In carrying out the process that is part of this invention the fineparticles of retorted oil shale are preferably burned in the fine shaleliftpipe with a substoichiometric amount of oxygen, i.e., insufficientoxygen to allow complete combustion of the char. An air to fuel ratio inthe range of from about 0.2 to about 0.9 would be suitable foroperation. This is to prevent excessive carbonate decomposition. Usuallytemperatures in both the fine shale liftpipe and the coarse preburnerare kept below about 1500° F.

To insure entrainment of the fine particles in the fine shale liftpipe aminimum gas velocity of about 4 to 6 feet per second is required toprevent choking, i.e., collapse of the solids in the pneumatic pipe.Preferably the velocity of the entraining gas is in the range of fromabout 10 feet per second to about 20 feet per second. In the preferredembodiment of this invention the fine particles have a residence time ofabout 1.5 to 2 seconds in the fine shale liftpipe which results in about25% of the carbonaceous residue being burned in that zone (air/fuel moleratio=0.25).

The coarse preburner is also operated in the substoichiometric mode andis usually designed to contain a bed of burning coarse materialfluidized by a flow of gas from below.

The flow of gas through the main combustion liftpipe must be sufficientto entrain both the fine and the coarse particles of shale. Usually agas velocity in the range of from about 50 feet per second to about 150feet per second is employed, with a preferred range of from about 80feet per second to about 100 feet per second. It should be noted thevelocity of the gas in the main combustion liftpipe is usually higherthan that in the fine shale liftpipe to insure entrainment of particlesentering from the coarse preburner. Combustion in the main liftpipe iscarried out with at least a stoichiometric amount of oxygen present, andusually an excess of oxygen is employed. In the preferred embodiment ofthis invention the velocity of the gas in the main combustion liftpipeis greater than that in the fine shale liftpipe.

As used in the specification and claims the phrase "terminal velocity"refers to the velocity of a gas stream necessary to entrain a given sizeof particles. Therefore, particles having a terminal velocity equal toor less than the velocity of a flow of gas will be pneumaticallyentrained.

The combustion device and process described herein has the advantage ofminimizing the height of the liftpipe required to completely burn theparticles of retorted oil shale. By preburning the coarse and fine shalein separate zones taking advantage of the relative sizes of both feeds,the total residence time required for combustion is satisfied in ashorter liftpipe.

Although the process of this invention is most advantageously used in anoil retorting process using recycled burned shale as the heat transfermaterial, the invention should not be limited to that retorting method.One skilled in the art can easily devise schemes by which other types ofheat transfer material, such as for example ceramic compositions, sand,alumina, iron, steel or the like, are employed. In such an instance theheat transfer material may be simply mixed with the coarse shalefeedstream to the combustor. Even in processes using spent shale as theprincipal heat transfer material, it is often necessary to addsupplemental heat transfer material to the systems. Likewise, it ispossible to devise other embodiments of the invention which couldutilize the hot flue gas in retorting the raw oil shale or even use thecombustor to heat water to produce steam.

What is claimed is:
 1. In a process for retorting oil shale using a heattransfer material heated by burning retorted oil shale containingresidual carbonaceous material, wherein said retorted oil shale containsboth fine and coarse grained material and at least part of said finegrained material is contained in a separate feedstream from the coarsegrained material, an improved process for burning the oil shalecomprising:(a) burning at least part of the residual carbonaceousmaterial in the fine grained material present in said separatefeedstream in a fine preburner by entraining said fine particles infirst entraining gas stream containing oxygen and having a velocitygreater than the terminal velocity of the fine particles; (b) burningpart of the carbonaceous residue in the coarse grained material in acoarse preburner containing less than a stoichiometric amount of oxygen;(c) mixing the entrained partially burned fine grained material from thefine preburner and the partially burned coarse grained material from thecoarse preburner in a second entraining gas stream having a velocitygreater than the terminal velocity of the mixture of fine and coarseparticles and containing at least a stoichiometric amount of oxygen; and(d) burning the carbonaceous residue remaining in the entrained fine andcoarse grained material in a vertical combustion zone through which theentraining gas is directed.
 2. Process of claim 1 wherein the finegrained material is partially burned in the fine preburner with asubstoichiometric amount of oxygen.
 3. Process of claim 1 wherein thetemperature of the material burned in the coarse preburner and the finepreburner does not exceed 1500° F.
 4. Process of claim 1 wherein thecoarse grained material in the coarse preburner is contained in afluidized bed.
 5. Process of claim 1 wherein the velocity of theentraining gas in the fine preburner is less than the velocity of thegas stream in the vertical combustion zone.
 6. Process of claim 5wherein the velocity of the gas stream in the vertical combustion zoneis in the range of from about 80 feet per second to about 150 feet persecond and the velocity of the entraining gas in the fine preburner isin the range of from about 10 feet per second to about 20 feet persecond.
 7. Process of claim 1 wherein burned oil shale is used as heattransfer material.